1. Field of the Invention
The present invention relates generally to a Broadband Wireless Access (BWA) communication system, and more particularly to a system and a method for assigning a sub-channel in a BWA communication system.
2. Description of the Related Art
Communication systems are now capable of providing voice services, data services, and various multimedia services. However, because the conventional voice-based communication system has a relatively small data transmission bandwidth and imposes high service charges on users, it is difficult to satisfy the increasing service demands of users.
In addition, with the development of communication industry and increase of user demands for an Internet service, it is more necessary to provide a communication system capable of satisfying the user demands.
Accordingly, a BWA communication system has been proposed, which has enough broadband to satisfy the user demands while efficiently providing an Internet service.
The BWA communication system integrates voice, low speed dates services and high speed data services with multimedia application services for high quality dynamic images, and then deliver these integrated services. The BWA communication system is capable of connecting to a Public Switched Telephone Network (PSTN), a Packet Data Serving Node (PDSN) network, an Internet network, an International Mobile Telecommunication (IMT)-2000 network, an Asynchronous Transfer Mode (ATM) network, etc., through wireless media using a broadband including 2 GHz, 5 GHz, 26 GHz, 60 GHz, etc. Further, the BWA communication system is capable of supporting a channel transmission rate of more than 2 Mbps.
Commonly, the BWA communication system is classified into a Broadband Wireless Local Loop (BWLL), a Broadband Radio Access Network (BRAN), and a high speed Wireless Local Area Network (WLAN) according to a terminal's mobility (stationary or mobile), communication environments (indoor or outdoor), and a channel transmission rate.
The radio access scheme of the BWA communication system has been standardized by an Institute of Electrical and Electronics Engineers (IEEE) 802.16 standardization group. The IEEE 802.16 standardization group is currently establishing an IEEE 802.16d standard and an IEEE 802.16e standard as a standard for providing a wireless broadband Internet service to stationary terminals or mobile terminals. In particular, research is being performed to support a high speed service capable of ensuring mobility and various levels of Quality of Service (QoS) in a BWA communication system such as a WLAN communication system and a wireless Metropolitan Area Network (MAN) communication system. Representative communication systems of the BWA communication system are the IEEE 802.16d communication system and the IEEE 802.16e communication system (hereinafter commonly referred to as IEEE 802.16d/e communication systems).
The IEEE 802.16d/e communication systems utilize an Orthogonal Frequency Division Multiplexing (OFDM) scheme/an Orthogonal Frequency Division Multiplexing Access (OFDMA) scheme in order to support a broadband transmission network for a physical channel of the wireless MAN system. However, the IEEE 802.16d communication system considers only a single cell structure and stationary Subscriber Station (SS), which means the system does not accommodate the mobility of the SSs at all. The IEEE 802.16e communication system accommodates the mobility of an SS in the IEEE 802.16d communication system. Herein, an SS having the mobility will be referred to as a Mobile Station (MS).
Because the IEEE 802.16d/e communication systems have a wide data transmission bandwidth as compared to radio technology for an existing voice service, they can transmit mass storage data in a short amount of time and efficiently use channels by sharing all user channels. Further, because the IEEE 802.16d/e communication systems ensure QoS, users can receive various services of different qualities according to the characteristics of a service.
In the IEEE 802.16d/e communication systems, because all users connected to a Base Station (BS) share and use a common channel and intervals, for which each user uses the channel, are assigned by the BS in each uplink and downlink frame, the BS must inform each user of uplink and downlink access information so that each user can separately use the channel.
Accordingly, the IEEE 802.16d/e communication systems classify channel information into uplink and downlink channel information, define information for each channel by Type, Length and Value (TLV), insert the defined information for each channel into a Downlink Channel Descriptor (DCD) message and a Uplink Channel Descriptor (UCD) message, and periodically transmit the DCD and UCD messages to all users, thereby informing SSs of characteristic information for channels.
FIG. 1 is a diagram schematically illustrating a conventional frame structure of an IEEE 802.16d or IEEE 802.16e communication system.
Referring to FIG. 1, a frame is classified into a downlink (DL) sub-frame 100 and an uplink (UL) sub-frame 140. The downlink sub-frame 100 includes a preamble field 110, an MAP field 120, and a downlink (DL) burst field 130. The preamble field 110 is a field through which synchronization signals, i.e., a downlink preamble sequence, are transmitted in order to acquire synchronization between a BS and SSs. The MAP field 120 is a field through which a downlink-MAP message is transmitted, and the DL burst field 130 is a field through which downlink data targeting the SSs are transmitted. The downlink sub-frame 100 includes sub-channels assigned through a normal sub-channel assignment scheme and a band Adaptive Modulation and Coding (AMC) sub-channel assignment scheme.
The uplink sub-frame 140 includes an uplink (UL) burst field through which uplink data targeting the BS are transmitted from the SSs.
The IEEE 802.16 d/e communication systems assign resources to each user through an uplink and a downlink by the sub-channel, which is a set of specific sub-carriers. The band AMC sub-channel assignment scheme adaptively changes a modulation technique and a coding technique according to wireless environments in order to improve the data transmission efficiency. Further, because it's a basic algorithm for the band AMC sub-channel assignment scheme is already known to those skilled to the art, a detailed description will be omitted.
The normal sub-channel assignment scheme may include a Partial Usage Sub-Channel (PUSC) assignment scheme, a Full Usage Sub-Channel (FUSC) assignment scheme, an optional FUSC assignment scheme, an AMC permutation assignment scheme, etc. For an uplink, a PUSC, an optional FUSC, an AMC permutation, etc., exist. All other sub-channel assignment schemes excluding the band AMC sub-channel assignment scheme basically assign sub-carriers randomly scattered in an entire frequency domain to one sub-channel, thereby causing each user receiving sub-channels to acquire frequency diversity gain.
In the IEEE 802.16 d/e communication systems, a BS assigns Channel Quality Indicator Channels (CQICHs) for reporting its own Channel Quality Indicator (CQI) to the SSs, and each of the SSs informs the BS of its own channel conditions through the assigned CQICH. The CQICH is a uplink channel used when the SS measures and compares reception qualities of downlink pilot channels received from BSs included in an active set, selects one BS from which the SS is to receive downlink packet data, and feedbacks the reception qualities of the pilot channels to the BS. That is, the CQICH represents a channel assigned to the SSs for the CQI.
More specifically, in the IEEE 802.16 d/e communication systems, the BS assigns the CQICH to all SSs, and each SS may transfer the CQI of 5 bits or 6 bits to the BS through the CQICH. For example, because the CQI of 6 bits, which may be transmitted from each SS through the CQICH corresponds to 26, it is possible to use the total 64 codewords. From among the 64 codewords, 32 codewords are used in order to report a Carrier-to-Interference Noise Ratio (CINR) value of a channel to the BS. With the exception of the already defined codewords used for other purposes of the other 32 codewords not being used for reporting the CINR value, seven codewords have not yet been currently defined.
If an SS has received a sub-channel through the normal sub-channel assignment scheme, the SS transitions an average CINR value for a whole bandwidth according to 32 (25) levels for a 5 bit encoding, and transmits encoding results to a BS through the CQICH.
If an SS has received a sub-channel through the band AMC sub-channel assignment scheme, the SS selects five bands having the best channel conditions, and transmits a differential value between CINR values for the selected bands to the BS. When the differential value received from the SS is 1, the BS determines that a CINR value of the current frame is larger than a CINR value of the previous frame. However, when the differential value is 0, the BS determines that the CINR value of the current frame is smaller than the CINR value of the previous frame.
When the BS assigns sub-channels through the band AMC sub-channel scheme, it is necessary to report CINR values for each band of the SSs to which the sub-channels are assigned.
FIGS. 2 and 3 are flow diagrams schematically illustrating conventional signal flows for sub-channel assignment between a BS and an SS in wither an IEEE 802.16d or IEEE 802.16e communication system. More specifically, FIG. 2 is a flow diagram illustrating a BS transmitting a CQI request to the SS and assigning a sub-channel to the SS, and FIG. 3 is a flow diagram illustrating an SS requesting the BS to assign a sub-channel by transmitting a CQI to the BS.
Referring to FIG. 2, the BS 220 transmits a CQI Report Request (REP-REQ) message to the SS 210 in step 201. The SS 210 transmits a Report Response (REP-RSP) message including its own CQI to the BS 220 in response to the REP-REQ message in step 202. After receiving the REP-RSP message including the CQI from the SS 210, the BS 220 assigns a sub-channel through the band AMC sub-channel assignment scheme in step 203. After receiving the sub-channel from the BS 220 through the band AMC sub-channel assignment scheme, the SS 210 selects five bands having the best channel conditions, and transmits a CQI including a differential value between CINR values for the five selected bands, i.e., a Band AMC Differential CQI, to the BS 220 from a frame, after transmitting the REP-RSP message in step 204.
Referring to FIG. 3, when the SS 310 is to receive a sub-channel from the BS 320 through the band AMC sub-channel assignment scheme, the SS 310 transmits the REP-RSP message including its own CQI to the BS 320 without reception of the REP-REQ message. That is, the SS 310 informs the BS 320 that the SS 310 is to first receive the sub-channel through the band AMC sub-channel assignment scheme in step 301. The BS 320 assigns the sub-channel through the band AMC sub-channel assignment scheme in step 302.
After receiving the sub-channel from the BS 320 through the band AMC sub-channel assignment scheme, the SS 310 selects five bands having the best channel conditions, and transmits a CQI including a differential value between CINR values for the five selected bands, i.e., a Band AMC Differential CQI, to the BS 320 from a frame after transmitting the REP-RSP message in step 303.
When a BS assigns a sub-channel through the normal sub-channel assignment scheme, the BS assigns the sub-channel by using the downlink MAP message of the MAP field 120 in FIG. 1 without using separate signals through the normal sub-channel assignment scheme, and an SS having received the sub-channel from the BS through the normal sub-channel assignment scheme reports an average CINR value for a whole bandwidth to the BS. That is, when the SS receives the sub-channel through the band AMC sub-channel assignment scheme, the SS selects five bands having the best channel conditions, and reports only a differential value between CINR values for the five selected bands. However, when the SS receives the sub-channel through the normal sub-channel assignment scheme, the SS reports the average CINR value for the whole bandwidth to the BS.
Herein, in the IEEE 802.16 d/e communication systems, it may be impossible for a BS to normally receive the REP-RSP message transmitted from an SS for reception of the sub-channel through the band AMC sub-channel assignment scheme. When the BS has not normally received the REP-RSP message from the SS, the BS may determine the differential value between the CINR values for the five bands having the best channel conditions, which is transmitted from a frame after the SS has transmitted the REP-RSP message, as the average CINR value for the whole bandwidth. Therefore, a problem may occur in the sub-channel assignment through the band AMC scheme and it may be difficult to provide a communication service to the SS. Further, the stability and reliability of the system may deteriorate.